1. Field of the Invention
The present invention relates to a semiconductor device which exhibits a low voltage drop in the conductive state. More particularly, it relates to a semiconductor device which has an impurity-doped polycrystalline semiconductor layer on one surface of a semiconductor substrate and which is formed with a P-N junction within the semiconductor substrate by the diffusion of an impurity in the polycrystalline semiconductor layer.
2. Description of the Prior Art
The diode is a semiconductor device comprising a semiconductor substrate which has an energy barrier, and a pair of main electrodes which are provided on both sides of the barrier of the semiconductor substrate. lWhen biased in the forward direction, by the application of a voltage between the pair of main electrodes, a low impedance is presented, so that current can easily flow; for a reverse bias, a high impedance is presented so that the flow of current is difficult. One of the important characteristics which the diode exhibits when biased in the forward direction is the characteristic of forward voltage drop.
In recent years, low voltages have been often used in computers and their terminals, equipment for automobiles, etc. Regarding diodes for such uses, those having low forward voltage drops have been especially desired. As a diode having a low forward voltage drop, the Schottky barrier diode has hitherto been known. This diode, however, has disadvantages in that the reverse blocking voltage is low and that the high temperature characteristic is poor. As a further disadvantage, it is difficult with present-day technology to fabricate a diode having a large current capability or a large Schottky barrier area with good reproducibility. On the other hand, a P-N junction diode can achieve a high blocking voltage and a large current capability comparatively easily, but it has the disadvantage that the forward voltage drop is ordinarily as great as 1-2 (V). For these reasons, a diode which has a low forward voltage drop and which has a high blocking voltage and large capability has been desired.
The thyristor is a semiconductor device comprising a semiconductor substrate which consists of at least four alternate P-N-P-N layers, a pair of main electrodes which respectively contact both the outer layers of the semiconductor substrate, and a trigger input such as a gate electrode, which supplies to the semiconductor substrate a trigger signal for shifting the current flow path between the pair of main electrodes from the non-conductive state to the conductive state. Also, this device has the same requirements as the diode where it is to be applied to a low voltage circuit.